Magnetic recording media which are generally and widely used these days typically employ as a binder a thermoplastic resin alone or in combination, such as a vinyl chloride-vinyl acetate type resin, a vinyl chloride-vinylidene chloride type resin, a cellulose resin, an acetal resin, and a urethane resin, or an acrylonitrile-butadiene resin. Such magnetic recording media have defects such as that as a wear resistance of a magnetic layer is poor, the tape path of magnetic tapes easily stains.
It is known that as a binder, a thermosetting resin such as a melamine resin or urea resin is used and that a binder of the above-described thermoplastic resin to which a binder cross-linkable by a chemical reaction such as an isocyanate compound or an epoxy compound is added is used. However, the use of the cross-linkable binder is accompanied by defects such as that (1) a resin solution having dispersed magnetic particles therein has poor storage stability, that is, pot life is short, physical properties of magnetic coating composition are not homogeneous and a magnetic tape cannot have homogeneous quality and that (2) after a magnetic coating composition is coated and dried, a heat treatment is necessary for hardening the coated layer. Thus, a long period of time is necessary for manufacturing the product.
In order to overcome the above defects, a method for preparing a magnetic recording medium using as a binder an oligomer and a monomer of acrylic acid ester type and hardening the binder by electron beam irradiation after drying is disclosed in U.S. Pat. Nos. 3,871,908 and 4,443,490, Japanese Patent Publication No. 12423/1972, and Japanese Patent Application (OPI) Nos. 15104/1972 and 77433/1975 (the term "(OPI)" used herein means a "published unexamined Japanese patent application").
Recently, a support of a magnetic recording medium has been required to be thin in order to provide long term recording and light weight thereof, and a magnetic recording medium has been required to be used under more harsh condition. A magnetic tape which is prepared by a conventional method using electron beam irradiation for hardening is often folded while the tape is running and the magnetic layer may easily separate from the support in a tape recorder, thereby staining the tape running path. In some cases, the magnetic layer may separate during the steps of smoothing treatment using calendering and slitting. Therefore, a sufficiently excellent magenetic recording medium has not been obtained so far.